1. Field of the Invention
This invention relates to the field of fiber optics, and more specifically to the selective excitation of predetermined modes within a multimode graded-index optical fiber.
2. Description of the Prior Art
It is known that selected modes of step-index optical fibers may be excited by directing a plane wave incident on the fiber end at an angle to the fiber axis. The mode excited is determined by control of the angle between the incident light beam and the fiber axis. This end-launch technique, when attempted with graded-index fibers, excites a spectrum of modes rather than a single one as is desired. This failure to achieve excitation of a single mode is documented for example by R. Olshansky, S. M. Oaks, and D. B. Keck, in Digest of Topical Meeting on Optical Fiber Transmission (Optical Society of America, Washington, D.C., 1977), paper TUE 5.
It is also known that selected "leaky" modes may be excited by directing a light beam through the side of a glass fiber, as taught for example by S. Zemon and D. Fellows, Applied Optics 15, 1936 (1976).
These leaky modes have propagation constants, .beta.=K COS .phi. smaller in value than kn.sub.cladded, the minimum propagation constant for guided modes excited via the end launch technique. The leaky modes suffer high loss, however, because a major portion of the energy propagates outside the fiber core.
Light wave refraction affects only the wavenumber component perpendicular to the index boundary, which under end launch conditions corresponds to the axial component. The wavenumber component parallel to an index boundary is unaffected by the boundary. This implies that under a side launch arrangement where all index boundaries occur parallel to the fiber axis, the axial component of the source field will not be perturbed by the index variations either in a graded- or step-index fiber.